Synchrotron infrared spectroscopy at megabar pressures: Vibrational dynamics of hydrogen to 180 GPa

We have developed new techniques for measuring infrared spectra at megabar pressures using synchrotron radiation and applied them to study the ${\mathit{Q}}_1$(1), ${\mathit{Q}}_1$(1)+${\mathit{S}}_1$(0), and ${\mathit{Q}}_{\mathit{R}}$(J) vibrational transitions of solid hydrogen to 180 GPa. The frequency difference between the ${\mathit{Q}}_1$(1) infrared and Raman vibrons increases from 3 ${\mathrm{cm}}^{\mathrm{-}1}$ (zero pressure) to 510 ${\mathrm{cm}}^{\mathrm{-}1}$ (180 GPa), including a dramatic increase in intermolecular coupling with pressure. A negative frequency shift is observed for the infrared vibron above 140 GPa. A significant increase in frequency and LO-TO splitting of the lattice phonon is also documented.